Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Misconceptions: Physics of Flight

On Wed, 11 Aug 1999, JACK L. URETSKY (C)1998; HEP DIVISION, ARGONNE NATIONAL LAB ARGONNE, IL 60439 wrote:

Hi romanza-
Since the air flowing over the top is moving faster, any given
parcel of air will travel further than air that is moving more slowly.
There is nothing wrong with this explanation. Where Anderson and
Eberhardt take exception, is the statement that the fast moving air
parcel must arrive at the trailing edge "at the same time as" the
parcels flowing along the bottom of the wing. That statement is
quantitatively incorrect, but that, in my opinion, is just a fine point
for pedants.

I must disagree, and so do several others who unlike myself actually are
aerodynamics professionals. In my experience, ANY attempt to correct a
widespread misconception can be dismissed as pedantic nitpicking. But
there is a difference between pedantic nitpicking and major
misconceptions.

The "difference in path-lengths" might SEEM to explain flight, but it is
fundamentally flawed because it does not explain *WHY* the air goes faster
over the top of the wing. It is also flawed because it leads to many
untrue predictions regarding airplane behavior, i.e, it causes
misconceptions and flawed reasoning.

The pressure difference across the wing as well as the lifting force
is far larger that could be explained by appeals to path-length
difference. If the lifting force was caused by path-length
differences, then the cross-section of a wing would resemble a whale!
(immense curved top, short flat bottom, extreme thickness in relation
to chord length.)

If path-length is the reason for lift, then in order to increase lift,
we would wish to increase path-length, and the best airfoil would have
a very large path length difference. In practice this does not work.

Adjacent parcels which are split by the wing's leading edge never
recombine behind the wing. They remain permanently separated
by quite a significant distance. This is obvious from wind-tunnel
photos and in computer simulations. Why does this occur? According
to "path-length difference", it doesn't.

If the wing is symmetrical and uncambered, the air STILL flows
faster over the top during flight. In other words, the Bernoulli
equation still applies, and the pressure-difference still arises, even
if the wing is as flat as a board. Therefor the "path-length
difference" does not apply to wings in general, and does not explain
how airplanes stay up there.

If the wing is cambered but is very thin, as with Hang GLiders,
paper airplanes, and the Wright Flyer, then the path length above
and below the wing is the same. Yet the air still insists on flowing
far faster over the top of the wing! Why? If we believe in the
"path length difference" explanation, then we cannot explain how the
Wright Flyer can work. We would predict that paper airplanes, since
their wings are not thick and since they lack the "path length
difference", cannot fly.

Reasoning from "path length difference", we would predict that
when an aircraft flys upside-down, the air which flows over the
upwards surface of the wing flows slower, and the lifting force
is directed towards the earth, and that inverted flight is impossible.
This does not occur. Why? WHy do aircraft behave opposite to what we
might expect from "path length" theory? What allows the difference in
air velocity to become reversed, even though the path length does not
change?

AIRFOIL MISCONCEPTIONS IN K-6 TEXTBOOKS
http://www.amasci.com/wing/airfoil.html

((((((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://www.amasci.com
EE/programmer/sci-exhibits science projects, tesla, weird science
Seattle, WA 206-781-3320 freenrg-L taoshum-L vortex-L webhead-L